In passive RFID systems the tags receive their power from the RF field transmitted by the reader. The efficiency of the power transfer from the reader to the tags is essential for reliable operation.
It is well know in the art, that radio wave propagation suffers from multipath effects. In the presence of reflections, the power received by the tags varies over location and frequency. If the received power at the tag falls below the sensitivity threshold of the tag, the tag will be missed from the inventory. Typical sensitivity of the tags today is −15 dBm to −20 dBm.
There are two commonly used methods to minimize the effect of multipath propagation in RFID systems, space diversity and frequency hopping.